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Abstract:

Disclosed herein is a method for preparing a near infrared absorbing
agent. The method includes admixing tungsten trioxide and a reducing
agent in water and allowing for a partial reduction of the tungsten
trioxides to yield the near infrared absorbing agent.

Claims:

1. A near infrared absorbing film, comprising: a polyacrylic acid matrix;
and a plurality of near infrared absorbing agents distributed within the
polyacrylic acid matrix, wherein the near infrared absorbing agents are
prepared by a method comprising: admixing tungsten trioxide and a
reducing agent in water, wherein the tungsten trioxide has a chemical
formula of WO.sub.3.H2O or WO.sub.3.2H2O; and allowing a
partial reduction of the tungsten trioxides in the water to proceed at a
temperature of 0-100.degree. C. and thereby yielding a reduced tungsten
oxide as the near infrared absorbing agent, wherein the valence of the
tungsten in the reduced tungsten oxide is from 5 to less than 6.

2. The film of claim 1, wherein the film has a near infrared absorbance
and a visible-light absorbance, and the near infrared absorbance is
greater than the visible-light absorbance.

3. The film of claim 1, wherein the film has an ultraviolet absorbance
greater than the visible-light absorbance of the film.

Description:

RELATED APPLICATIONS

[0001] This is a divisional application of patent application Ser. No.
12/648,678 filed on Dec. 29, 2009, now allowed, the entirety of which is
incorporated herein by reference.

BACKGROUND

[0002] 1. Field of Invention

[0003] The present invention relates to a near infrared (NIR) absorbing
agent.

[0004] 2. Description of Related Art

[0005] Tungsten bronzes MxWO3 (M=Li.sup.+, Na.sup.+, K.sup.+,
Rb.sup.+, Cs.sup.+) are known to possess a satisfactory absorption of NIR
light (having a wavelength between about 750 nm to about 1400 nm) while
retaining a high transmittance of visible light (having a wavelength
between about 380 nm to about 750 nm).

[0006] However, conventional methods for preparing tungsten bronze usually
involve an annealing step at a temperature higher than 300° C. to
produce the crystalline products that exhibit desirable NIR absorption.
High temperature annealing would usually raise the manufacturing cost and
is undesirable.

SUMMARY

[0007] The following presents a simplified summary of the disclosure in
order to provide a basic understanding to the reader. This summary is not
an extensive overview of the disclosure and it does not identify
key/critical elements of the present invention or delineate the scope of
the present invention. Its sole purpose is to present some concepts
disclosed herein in a simplified form as a prelude to the more detailed
description that is presented later.

[0008] In one aspect, the present invention is directed to a method for
preparing a near infrared (NIR) absorbing agent.

[0009] According to one embodiment of the present invention, the method
includes the step(s) as follows. Tungsten trioxide and a reducing agent
are admixed in water to allow a partial reduction of the tungsten
trioxides thereby yielding the near infrared absorbing agent.

[0010] In another aspect, the present invention is directed to a method
for preparing a near infrared absorbing film.

[0011] According to one embodiment of the present invention, the method
includes the steps as follows. First, near infrared absorbing agents
prepared in accordance with the above-mentioned aspect/embodiment(s) of
the present disclosure are dissolved in a polyacrylic acid aqueous
solution. Thereafter, the polyacrylic acid aqueous solution is coated on
a substrate and a film formation is allowed to take place thereby
producing the near infrared absorbing film.

[0012] In yet another aspect, the present invention is directed to a near
infrared absorbing film.

[0013] According to one embodiment of the present invention, the film
comprises a polyacrylic acid matrix and a plurality of near infrared
absorbing agents distributed therewithin. The plurality of near infrared
absorbing agents is prepared in accordance with the above-mentioned
aspect/embodiment(s) of the present disclosure. The film has a near
infrared absorbance and a visible-light absorbance, and the near infrared
absorbance is greater than the visible-light absorbance.

[0014] Many of the attendant features will be more readily appreciated as
the same becomes better understood by reference to the following detailed
description considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present description will be better understood from the
following detailed description read in light of the accompanying
drawings, wherein:

[0016]FIG. 1 is an absorbing spectrum according to a comparative example;

[0017]FIG. 2 is an absorbing spectrum according to working example 1A of
the present disclosure;

[0018]FIG. 3 is an absorbing spectrum according to working example 1B of
the present disclosure;

[0019]FIG. 4 is an absorbing spectrum according to working examples 2A-2C
of the present disclosure;

[0020]FIG. 5 is an absorbing spectrum according to working examples 2D-2F
of the present disclosure;

[0021]FIG. 6 is an absorbing spectrum according to working examples 2G-2I
of the present disclosure;

[0022]FIG. 7 is an absorbing spectrum according to working examples 3A-3C
of the present disclosure;

[0023]FIG. 8 is an absorbing spectrum according to working examples 3D-3F
of the present disclosure; and

[0024]FIG. 9 is an absorbing spectrum according to working examples 3G-3I
of the present disclosure.

DETAILED DESCRIPTION

[0025] The detailed description provided below in connection with the
appended drawings is intended as a description of the present examples
and is not intended to represent the only forms in which the present
example may be constructed or utilized. The description sets forth the
functions of the example and the sequence of steps for constructing and
operating the example. However, the same or equivalent functions and
sequences may be accomplished by different examples.

[0026] In one aspect, the present invention is directed to a method for
preparing a near infrared (NIR) absorbing agent.

[0027] According to one embodiment of the present invention, the method
includes the step(s) as follows. Tungsten trioxide and a reducing agent
are admixed in water to allow a partial reduction of the tungsten
trioxides thereby yielding the near infrared absorbing agent.

[0028] According to the embodiment of the present disclosure, the tungsten
trioxide may be hydrated tungsten trioxide, examples of which may
include, but are not limited to tungstite (WO3.H2O), meymacite
(WO3.2H2O) and hydrotungstite (H2WO4).

[0029] In optional embodiments, the tungsten trioxide is in a form of
powders with a diameter of about 50 to about 500 nm. For example, the
diameter of the tungsten trioxide may be about 50, 55, 60, 70, 80, 90,
100, 150, 200, 250, 300, 350, 400, 450, or 500 nm.

[0030] In optional embodiments, the tungsten trioxide is hydrated tungsten
trioxide prepared by a method comprising the steps as follows. In a
container charged with nitrogen, about 0.2-0.5 M
Na2WO4.2H2O aqueous solution and HCl are admixed to form a
reaction system, wherein HCl is added in an amount such that the reaction
system has a pH of about 1 to about 7. The reaction system is maintained
at a temperature of about -10° C. to about 10° C. to effect
the formation of hydrated tungsten trioxide.

[0031] Specifically, the pH of the reaction system can be maintained at
about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5 or 7. The
temperature of the reaction system may be kept at about -10, -9, -8, -7,
-6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10° C. The
volumetric molar concentration of the Na2WO4.2H2O present
in the aqueous solution may be about 0.2, 0.3, 0.4, or 0.5 M.

[0032] Generally, the as-produced powders would have a diameter of about
50 to about 500 nm. As such, it is suitable to be used to prepare the NIR
absorbing agent in accordance with the present aspect.

[0033] In the present disclosure, the term "partial reduction" refers to a
condition where only a portion of the tungsten trioxides are reduced to
tungsten oxides while the other portion of the tungsten trioxides are not
reduced. In this case, the valance m of the (non-reduced) tungsten
trioxide is 6+, while the valance n of the reduced tungsten oxide is
5≦n≦6. For example, valance n of the reduced tungsten oxide
may be about 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, or 5.9. In some
embodiments, valance n of the reduced tungsten oxide may be about 5,
5.25, 5.33, 5.5, 5.66, or 5.75.

[0034] In the present disclosure, the product of the partially reduction
process of the tungsten trioxides is a mixture of the tungsten (VI)
trioxides (or tungsten trioxides, for the sake of brevity) and reduced
tungsten oxides. In this context, the mixture may also be referred to as
mixed-valent tungsten (VI/V) oxides.

[0035] It is known that tungsten (VI) trioxides may exhibit desirable UV
absorbance, whereas the visible light absorbance and NIR absorbance
thereof are less than 10%, respectively. In fact, tungsten (VI) trioxides
are almost transparent to visible and NIR lights. On the other hand, the
reduced tungsten oxides, similar to tungsten bronzes (MxWO3,
where M=Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+, Cs.sup.+), may exhibit NIR
absorbing efficacy. As such, the mixed-valent tungsten (VI/V) oxides may
exhibit the UV and NIR absorbing efficacies at the same time.

[0036] Hence, according to the principles and spirits of the present
disclosure, it is important to control the reducing condition so as to
yield a near infrared absorbing agent that may exhibit both the UV and
NIR absorbing efficacies. Generally, the reducing condition may be
influenced by several factors, including but not limited to: the species,
volume and concentration of the reducing agent, the reaction pH, and the
concentration of the tungsten trioxide to be reduced.

[0038] In an illustrative embodiment where sodium borohydride is used as
the reducing agent, a weight ratio of the tungsten trioxide to the sodium
borohydride is about 5:1 to 20:1. For example, the weight ratio may be
about 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1,
17:1, 18:1, 19:1 or 20:1.

[0039] In another illustrative embodiment where ethanol is used as the
reducing agent, a weight ratio of the tungsten trioxide to the ethanol is
about 1:3 to 1:5. In particular, the weight ratio may be about 1:3.1,
1:3.2, 1:3.3, 1:3.4, 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9, 1:4, 1:4.1,
1:4.2, 1:4.3, 1:4.4, 1:4.5, 1:4.6, 1:4.7, 1:4.8, 1:4.9, or 1:5.

[0040] The method of the present aspect may be carried out at ambient
temperatures, but the present method is not limited thereto. Said ambient
temperatures are generally in a range of about 23-27° C. It should
be noted that the reaction system uses water as its main solvent.
Accordingly, the method may be carried out at a temperature range of
about 0-100° C.

[0041] Furthermore, the method according to the present aspect is carried
out without the application of an electric field. Hence, the method does
not involve an electrochemical reaction.

[0042] It is also noted that the mixed-valent tungsten (VI/V) oxides thus
obtained may exhibit both NIR absorbance and ultraviolet (UV) absorbance
without the conventional annealing step. Conventional annealing step is
usually carried out at a temperature higher than about 300° C. so
as to obtain a crystalline product. In some embodiments of the present
disclosure, the mixed-valent tungsten (VI/V) oxides may not undergo a
heating process that is higher than 100° C.

[0043] The NIR absorbing agent (i.e., the mixed-valent tungsten (VI/V)
oxides) thus obtained may be dissolved in water without being dissolved
in other organic solvent first. Accordingly, the NIR absorbing agent is
suitable for use in the preparation of an NIR absorbing film.

[0044] Therefore, in another aspect, the present invention is directed to
a method for preparing a near infrared absorbing film.

[0045] According to one embodiment of the present invention, the method
includes the steps as follows. First, near infrared absorbing agents
prepared in accordance with the above-mentioned aspect/embodiment(s) of
the present disclosure are dissolved in a polyacrylic acid aqueous
solution. Thereafter, the polyacrylic acid aqueous solution is coated on
a substrate and a film formation is allowed to take place thereby
producing the near infrared absorbing film.

[0046] It should be noted that the NIR absorbing agents according to the
first aspect of the present disclosure may be dissolved in water without
being dissolved in other organic solvent first. Hence, in some
embodiments of the present disclosure, the polyacrylic acid aqueous
solution does not include any other organic solvent for dissolving the
plurality of near infrared absorbing agents. It should be noted that, the
polyacrylic acid in the solution is not used as a solvent for dissolving
the NIR absorbing agents. It is of course possible that some variations
may be made to these embodiments by adding additional organic components
in the polyacrylic acid aqueous solution. However, as long as the
presence of these organic components does not facilitate the dissolvation
of the NIR absorbing agents in the polyacrylic acid aqueous solution,
these variations do not depart from the spirit or scope of the present
disclosure.

[0047] As described hereinbefore, the near infrared absorbing agent per se
may exhibit both NIR and UV absorbing efficacies without the conventional
annealing step. Accordingly, the UV absorbing film thus prepared may also
exclude the conventional annealing step. However, it is possible to
perform a heating process at a temperature lower than 100° C.
during the film forming step to facilitate the removal of the solvent.

[0048] In yet another aspect, the present invention is directed to a near
infrared absorbing film.

[0049] According to one embodiment of the present invention, the film
comprises a polyacrylic acid matrix and a plurality of near infrared
absorbing agents distributed therewithin. The plurality of near infrared
absorbing agents is prepared in accordance with the above-mentioned
aspect/embodiment(s) of the present disclosure. The film has a near
infrared absorbance and a visible-light absorbance, and the near infrared
absorbance is greater than the visible-light absorbance.

[0050] In the present disclosure, a term "total value" is used to describe
the NIR absorbing and visible-light absorbing properties of a film. Total
value is the value of the sum of the value of the NIR absorbance of a
film and the value of the visible-light transmittance of the film.

[0051] Generally, light transmittance (T %) within a specified wavelength
range can be determined by a spectrometer, and light absorbance within
the same range is obtained by subtracting the light transmittance from
100%. Hence, for example, the visible-light absorbance (VIS Abs. %) of a
film is obtained by subtracting a visible-light transmittance (VIS T %)
thereof from 100%.

[0052] Accordingly, when a film has an NIR absorbance greater than the
visible-light absorbance, the relationship of the NIR absorbance and
visible-light absorbance may be expressed as the inequality equation I:

NIR Abs. %>VIS Abs. % Equation I.

The equation I is still true if "100%" is added to both sides of the
inequality equation I:

100%+NIR Abs. %>100%+VIS Abs. % Equation II.

The equation II is also true if "-VIS Abs. %" is added to both sides of
the inequality equation II:

(100%-VIS Abs. %)+NIR Abs. %>100% Equation III.

Equation III can be rewritten into equation IV:

VIS T %+NIR Abs. %>100% Equation IV.

Hence, it is concluded that when a film has an NIR absorbance greater
than the visible-light absorbance, the sum of the VIS T % and NIR Abs. %
of the film is greater than 100%. That is, the total value of the film is
greater than 100.

[0053] Optionally, in some embodiments, the UV absorbance of the film may
be greater than the visible-light absorbance of the film.

[0054] Some working examples according to embodiments of the present
invention are provided hereinafter. The tungsten trioxides used in these
examples were hydrated tungsten trioxide prepared by the method described
hereinabove. Specifically, about 0.03 mole Na2WO4.2H2O and
about 2.5 ml of HCl were admixed with 100 ml of water in a container
charged with nitrogen to form a reaction system. The reaction system was
maintain at a temperature of about -5° C. to about 5° C. to
effect the formation of hydrated tungsten trioxide as white precipitate.
The resulting precipitate was collected and washed with 40 ml of iced
water to yield the product.

[0055] In some examples, the product can be re-suspended in water for use
in the following partially reduction process. Alternatively, in some
other examples, the product can be dried in vacuo to yield the tungsten
trioxide powders, which can be later suspended in water for use in the
following partially reduction process.

[0056] The hydrated tungsten trioxides thus prepared were partially
reduced in accordance with the aspect and embodiments described
hereinabove to yield the NIR absorbing agents. The NIR absorbing agents
were further used for making the NIR absorbing films in accordance with
the aspect and embodiments described hereinabove. Spectrometer (Hitachi
U-4100 spectrometer) was used to measure the light transmittance of the
films in the wavelength range of 280-1100 nm so as to determine the UV
(wavelength 280-380 nm) transmittance, visible-light (wavelength 380-750
nm) transmittance, and NIR (wavelength 750-1100 nm) transmittance of each
film, respectively.

[0057] The amounts of the reducing agent and/or the polyacrylic acid used
in each example and the results thereof are summarized in hereinafter.

COMPARATIVE EXAMPLES

Comparative Example 1

[0058] The hydrated tungsten trioxides prepared by the method described
hereinabove were re-suspended in about 5 ml of water. The suspension
(without being reduced) was then dissolved in about 2.86 g of about 35 wt
% polyacrylic acid (PAA) solution. The solution was stirred at a
temperature of about 75° C. for about 60 minutes. Thereafter,
about 1 ml of the solution was sprayed over a glass substrate, and the
substrate was baked at about 60° C. for about 60 minutes to allow
the film formation. The film was analyzed with the spectrometer, and the
light absorbing spectrum of the film of comparative example 1 is shown in
FIG. 1. As can be seen in FIG. 1, the NIR transmittance of the film is
almost 100%, and hence, the film of comparative example 1 exhibited
substantially no NIR absorbing efficacy. The analysis also showed that a
UV absorbance of the non-reduced film is about 73%, and the visible-light
transmittance thereof is about 99%.

Comparative Example 2

[0059] Powders of WO3 (about 1.16 g, 5 mmol) and about 0.60 g of NaOH
(15 mmol) were ground together sufficiently to get Na2WO4/NaOH
mixture. The mixture and about 50 ml distilled water were put into a 200
ml beaker and kept under constant stirring on a magnetic stirrer.
Afterward, about 0.47 g (15 mmol) of NaBH4 was dissolved in this
solution, in which pH was about 13.5 that could suppress the rate of
evolution of hydrogen from NaBH4, and reduction of Na2WO4
did not occur kinetically. Hydrochloric acid (1.5 mol/l) was re-added
into the beaker at a slow speed of about 0.5 drop per second to induce
the reduction till pH of the reaction solution decreased to about 6.5.
This procedure resulted in a dark brown gel in a blue sol medium. The gel
was allowed to settle for 3 hours and then washed with warm distilled
water three times. Finally, the solid product was filtered, washed with
ethanol and dried in vacuum dry oven at ambient temperature to afford
about 0.58 g of powders. The resulting powders were added into about 2.86
g of about 35 wt % PAA aqueous solution. However, it is observed that the
powders were hardly dissolved in the PAA solution. Hence, the resulting
powder is not suitable to be used in the method for preparing a near
infrared absorbing film according to the present disclosure. Moreover,
the XRD analysis showed that the resulting powders were amorphous.

Example 1

Working Example 1A

[0060] The hydrated tungsten trioxides prepared by the method described
hereinabove were re-suspended in about 5 ml of water. About 0.5 ml of
about 1 wt % NaBH4 aqueous solution was added into the suspension, and
the suspension was stirred at 400 rpm for about 10 minutes so that the
tungsten trioxides are partially reduced. Thereafter, about 8.22 g of PAA
and about 22 ml of water were added into the solution, and the solution
was stirred at 75° C. for about 60 minutes. The film formation
step was carried out in the way similar to the comparative example 1.

Working Example 1B

[0061] The hydrated tungsten trioxides prepared by the method described
hereinabove were re-suspended in about 5 ml of water and vortexed at
about 2000 rpm for about 10 minutes. After the removal of the water in
the upper layer, 40 ml of iced water was added and vortexed at about 2000
rpm for about 10 minutes. The resulting precipitate was dried in vacuo in
a vacuum dry oven for a day to yield the powders. 0.1 g of the resulting
powders was added into about 1 ml of about 1 wt % NaBH4 aqueous
solution, and the reaction system was stirred at about 400 rpm for about
10 minutes so that the tungsten trioxides are partially reduced.
Thereafter, about 8.22 g of PAA and about 22 ml of water were added into
the solution, and the solution was stirred at about 75° C. for
about 60 minutes. The film formation step was carried out in the way
similar to the comparative example 1.

[0062] The light absorbing spectrums of the film of working examples IA
and IB are shown in FIG. 2 and FIG. 3, respectively, and the results are
summarized in Table 1.

[0063] Working examples 2A-2I were similar to working example 1B except
the volume of the about 1 wt % reducing agent (NaBH4) and water were
different, and further, about 2.86 g of about 35 wt % PAA solution was
used for the film formation. In addition, the films of examples 2A, 2D
and 2G were made from about 0.3 ml of film forming solution; the films of
examples 2B, 2E and 2H were made from about 0.5 ml of film forming
solution; whereas the films of examples 2C, 2F and 2I were made from
about 1.0 ml of film forming solution. The amount of the reducing agent
and the results of the light absorbing analysis are summarized in Table
2, and the light absorbing spectrums of the film of working examples
2A-2I are shown in FIG. 4, FIG. 5, and FIG. 6.

[0064] Working examples 3A-3I were similar to working example 1B except
the volume of the about 35 wt % PAA solution and water were different. In
addition, the films of examples 3A, 3D and 3G were made from about 0.3 ml
of film forming solution; the films of examples 3B, 3E and 3H were made
from about 0.5 ml of film forming solution; whereas the films of examples
3C, 3F and 3I were made from about 1.0 ml of film forming solution. The
volume of the PAA solution and the results of the light absorbing
analysis are summarized in Table 3, and the light absorbing spectrums of
the film of working examples 3A-3I are shown in FIG. 7, FIG. 8, and FIG.
9.

[0065] Working examples 4A-4D employs ethanol as the reducing agent.
Specifically, about 0.03 mole Na2WO4.2H2O and about 2.5 ml
of HCl were admixed with about 75 ml of water and about 25 ml of ethanol
in a container charged with nitrogen to form a reaction system. The
reaction system was maintain at a temperature of about -5° C. to
about 5° C. to effect the formation of partially reduced tungsten
trioxide as white precipitate. The resulting precipitate was collected
and washed with about 40 ml of iced water to yield the product. About
8.22 g of PAA was dissolved in about 22.8 ml of water, and then admixed
with the resulting product. The system was vibrating in a water bath of
about 100° C. for about 1 hour. Thereafter, about 0.5 ml (Examples
4A and 4B) and 1 ml (Examples 4C and 4D) of the solution were
respectively sprayed over a glass substrate, and the substrate was baked
at about 60° C. for about 60 minutes to allow the film formation.
The film of examples 4B and 4D were further irradiated by a UV lamp
(wavelength: 365 nm) for about 60 minutes. The results of the light
absorbing analysis are summarized in Table 4.

[0066] It will be understood that the above description of embodiments is
given by way of example only and that various modifications may be made
by those with ordinary skill in the art. The above specification,
examples and data provide a complete description of the structure and use
of exemplary embodiments of the invention. Although various embodiments
of the invention have been described above with a certain degree of
particularity, or with reference to one or more individual embodiments,
those with ordinary skill in the art could make numerous alterations to
the disclosed embodiments without departing from the spirit or scope of
this invention.